1. Field of the Invention
The present invention is broadly concerned with improved, high capacity die assemblies for use with single or twin extruders. More particularly, the invention is concerned with such die assemblies, extruders including the die assemblies, and corresponding methods wherein the die assemblies preferably include a tubular section having a progressively diverging bore, an axially rotatable flighted screw section also progressively diverging within the tubular section, and a die unit associated with the head and screw sections. The diverging construction of tubular and screw sections allows the use of larger area dies having a greater number of die openings therein to thus increase extrusion rates.
2. Description of the Prior Art
Extrusion processing of comestible products such as human foods and animal feeds has long been practiced and is a highly developed art. In general terms, food extruders of the single or twin screw variety are employed, having elongated, tubular barrels with inputs adjacent one end thereof and restricted orifice dies at the outlet thereof, and one or two helically flighted, rotatable screws within the barrel. In many instances steam is injected into the barrel during processing. An apertured die plate is operably coupled with the outlet end of the barrel in order to form the product as it emerges from the extruder. Depending upon the selected extrusion conditions, the final products may be fully or partially cooked, and can have varying degrees of expansion.
Many commercial extruders are designed with converging terminal screw sections sometimes referred to as “cone nose” sections. These screw sections are housed within a complementally converging barrel section. These converging extruder end assemblies are provided in order to increase the pressure and shear conditions within the extruder just upstream of the final extrusion die. Typical examples of these designs are found in U.S. Pat. Nos. 4,118,164 (single screw) and 4,875,847 (twin screw).
Referring to FIG. 18, conventional prior art food extrusion assemblies are generally made up of a serially interconnected preconditioner 10 and extruder 11, the latter having a multiple section barrel 12 terminating in a restricted orifice die 13, and one or two internal, helically flighted, axially rotatable screw(s) 14. The screw 14 include an elongated central shaft 15 with helical flighting 16 along the length thereof. In many cases, the terminal head and screw sections 17 and 18 are of converging, frustoconical design in order to increase the pressure and shear conditions within the barrel just upstream of die 13.
In operation, a comestible material (usually a mixture of ingredients including quantities of protein, starch and fat) is fed into preconditioner 12 where it is initially moisturized by the addition of steam and/or water and heated to partially cook the material. The preconditioned material is then delivered to the extruder 11 where the action of the rotating screws(s) 14 serves to convey the material toward and through die 13. During this conveyance, the material is subjected to increasing levels of temperature, pressure and shear, in order to cook the material to the desired degree. As the material emerges from the die 13, it is formed as a final product and may undergo expansion as a result of flash-off of moisture from the material. The degree of expansion is a controlled phenomenon, and is influenced by the amount of energy imparted to the material within the barrel 12 and the geometry of the final die 13.
These food extruder systems have been used for decades to produce a wide variety of human foods and animal feeds. However, it has been found that the frustoconical end configuration of the terminal barrel and screw sections 17 and 18 may limit the rate of production achievable with such extrusion assemblies. Specifically, owing to the fact that the terminal barrel and screw sections converge, the die 13 necessarily has a reduced surface area, and hence can only have a certain number of restricted die openings therein. Indeed, this limitation on the number of available die openings is the limiting factor in production rates for some products such as small diameter or micro-aquatic feeds.
For example, using a typical Wenger Model X165 single screw extruder for the production of micro-aquatic feeds having a diameter of up to 2 mm., the maximum throughput is on the order of 1-1.5 tons/hr., and this rate limitation is attributable to the presence of only a small number of die holes.
Other conventional extruder designs are disclosed in U.S. Pat. Nos. 3,728,053; 3,904,341; 4,346,652; 4,352,650; 4,400,218; 4,422,839; 4,836,460; 5,458,836; 6,074,084; 6,331,069; 6,491,510; 7,101,166; Japanese Patent No. JP 55013147; Non-Patent Literature: Effects of Die Dimensions on Extruder Performance; Sokhey A S; Ali Y; Hanna M A; Foodline; Influence of Extrusion Conditions on Extrusion Speed, Temperature, and Pressure in Extruder and on Pasta Quality; Abecassis, J.; Abbou, R; Food Sci.&Tech. Abs; and Barrel-Valve Assembly: Its Influence of Residence Time Distribution and Flow Pattern in a Twin-screw Extruder; Liang, M.; Hsieh, F; AGRICOLA.